Conventionally, a quartz glass has been used for an arc tube material of high-pressure electric discharge lamps, but, in recent years, high-pressure electric discharge lamps using translucent ceramics for the arc tube material have been developed as products. In the high-pressure electric discharge lamps, particularly, metal halide lamps, when the arc tube material is a quartz glass, the quartz glass and metal halide as a light emitting substance gradually react during lighting and create the cause of degradation of the life characteristic. However, when the arc tube material is translucent ceramic, since it hardly reacts with the metal halide, a better life characteristic than that of the arc tube made of the quartz glass is obtained and the arc tube can be made compact, thereby creating a possibility of producing a lamp having good luminous efficiency and color rendering property. For such reasons, in recent years, electric discharge lamps using translucent ceramics for the arc tube material have been put into practical applications.
As a conventional example of the sealing structure of the arc tube of an electric discharge lamp using a ceramic tube, one shown in FIG. 1 and disclosed in Japanese Patent Application Laid-Open No. 6-196131 (1994) has been known. The arc tube is constructed by a wide tube 11 made of translucent ceramic and narrow tubes 12 made of the same translucent ceramic and provided at both ends of the wide tube 11. An electricity introducing member constructed by a first electricity introducing member 24 and a second electricity introducing member 27 is inserted into the narrow tube 12. The first electricity introducing member 24 is formed of a halogen-resistant electricity introducing member, such as molybdenum and cermet. The second electricity introducing member 27 is formed of an electricity introducing member having no halogen resistance, such as niobium. The first electricity introducing member 24 and the second electricity introducing member 27 were butt-welded at a welding section 26. Moreover, an electrode is constructed by an electrode core 21 butt-welded to the first electricity introducing member 24 at a welding section 25 and a coil 20 wound round the electrode core 21.
The first electricity introducing member 24 holding the electrode core 21, the second electricity introducing member 27 and the narrow tube 12 are airtightly sealed with a halogen-resistant sealing glass 30. The second electricity introducing member 27 is protected from halogen corrosion by covering its portion inserted into the narrow tube 12 with the halogen-resistant sealing glass 30. Furthermore, a part of the first electricity introducing member 24 is also covered with the sealing glass 30.
In the electric discharge lamp using translucent ceramic, it is difficult to highly reliably form the sealed sections of the electricity introducing member at the ends and the difficulty particularly increases as the diameter of the end becomes larger, and thus the conventional structure as described above has a drawback that it is not applicable to electric discharge lamps of large electric power consumption. In general, in an electric discharge lamp, the larger the electric power consumption is, the larger the current flows, but it is necessary to increase the diameter of the electrode core 21 constituting the electrode for a flow of a large current. In the above-described structure, if the diameter of the electrode core 21 is to be increased, the inner diameter of the narrow tube 12 must be increased.
However, when the inner diameter of the narrow tube 21 is increased, the gap between the electricity introducing member (the first electricity introducing member 24 and second electricity introducing member 27) and the narrow tube 12 becomes larger, resulting in difficult sealing. In other words, since the large gap between the electricity introducing member and the narrow tube 12 is filled with the sealing glass 30, a leakage of airtightness from the thicker layer of the sealing glass 30 is likely to occur.
In general, the thinner the layer thickness of the sealing glass 30, the higher the heat resistance of the sealed section, but, if the conventional structure is applied to a lamp of large electric power consumption, the layer thickness is unavoidably increased, resulting in problems that the narrow tube 12 will crack during sealing and, even when sealing is satisfactorily achieved, a leakage of airtightness from the layer of the sealing glass 30 will occur at an early stage due to the heat cycle by switching the lamp on and off.
In order to avoid such problems, it can be considered to increase the inner diameter of the narrow tube 12 and the diameter of the electricity introducing member. In this method, however, satisfactory sealing can not be achieved because of a difference in the coefficients of linear expansion between the different materials of the electricity introducing member and the narrow tube 12. Therefore, the conventional structure can be applied to lamps whose narrow tube 12 has an inner diameter smaller than 1.3 mm and electric power consumption is relatively small, not more than 150 W, but it cannot be applied to lamps of electric power consumption of more than 150 W.
For the sealing glass 30, two kinds of materials have been used conventionally: a material having a composition of Al2O3: 30 weight %, SiO2: 40 weight % and Dy2O3: 30 weight %, which has poor retention of airtightness but has excellent halogen resistance, for a side facing the discharge space; and a material having a composition of Al2O3: 13 weight %, SiO2: 37 weight % and Dy2O3: 50 weight %, which has poor halogen resistance but has excellent retention of airtightness, for a side that does not face the discharge space. Since such two kinds of materials are used for the sealing glass 30, it is necessary to divide the sealing process into two stages, resulting in problems that the sealing process becomes complicated and unsuitable for mass-production.
The present invention has been made on the basis of the above circumstances, and its object is to provide an electric discharge lamp capable of increasing the reliability of the sealed section of an arc tube for discharge and improving the life characteristic.
Another object of the present invention is to provide an electric discharge lamp having the sealed section of good reliability, excellent life and large electric power consumption.
Still another object of the present invention is to provide an electric discharge lamp capable of improving the reliability of the sealed section and the mass-productivity of the sealing process.